This application is directed to a method of coating articles such as superalloy turbine parts with a protective coating which may contain several elements, but will generally contain a significant percentage of aluminum.
The life of gas turbine parts which are subjected to the passage of the hot gas stream can be extended by bonding a coating of an oxidation resistant material such as an aluminide material to the surface of the part. Considerable research effort has been directed to the selection of the oxidation resistant material and the process for application of the material. This has resulted in the evolution of gas turbine blades which now have a life in the order of thousands of hours (when coated) compared to a life of only a few hundred hours if the same blades were operated in the hot gas stream in an uncoated condition.
Gas turbine parts which are subjected to the passage of the hot gas stream are required to operate in a hostile environment. Not only must these parts, typically turbine blades and vanes, withstand the intense heat produced in the hot gas stream, but both turbines and vanes are required to deflect the hot gas stream to enable the turbine engine to extract energy from the hot gas stream.
Turbine blades are also subjected to substantial centrifugal forces during operation; the coating must not migrate, peel or crack in the presence of any of the above forces during operation of the turbine.
The hot turbine parts are also subjected to the passage of a hot gas stream which usually contains corrosive materials such as sodium, potassium, sulphur and vanadium which may be present in the turbine fuel. Sometimes the turbine may ingest other substances in the intake air such as seawater salt which when heated can become very corrosive.
Similarly the atmosphere contains particles of dust and other foreign particles which when ingested in the turbine and accelerated in the hot gas stream tend to xe2x80x9csand blastxe2x80x9d the surfaces of the turbine blades and vanes to erode the surface thereof.
In addition to the above forces, each turbine blade is subjected to pulsations in pressure during turbine operation which cause deflections and vibrations in the blade which the coating must also endure without peeling, spalling or cracking during the lifetime of the blade.
In the past, turbine parts which must operate in the hot gas stream have been successfully coated with selected metals or metal oxides applied to the surface of the superalloy by various techniques which have evolved over the past forty years. Some methods of coating involve vapour deposition; others utilize a pack cementation process; still others use a slurry deposition process in which a carrier in which particles of selected metals are held in suspension so that the mixture of carrier and metallic particles may be applied to a substrate to form a coating on the substrate.
In a slurry process a powder composed of selected metallic particles is typically suspended in a carrier which performs the function of a fugitive binder, and which carrier functions to carry the suspended metallic particles to the cleaned surface of the substrate to be coated. If the selection of components of the mixture is done correctly, the carrier and suspended particles will form a uniform coating on the surface of the substrate and the carrier will temporarily bind the particles of the metallic material to the surface of the substrate (see U.S. Pat. No. 3,102,044 for example). The carrier is then driven off by some means, usually by heating the substrate to a predetermined temperature. If necessary, the coated substrate may then be subjected to additional heat treatment procedures to bond a continuous uninterrupted coating of a protective material to (the surface of) the substrate.
The carrier for the metallic or metallic oxide particles must be capable of keeping the metallic or metallic oxides in suspension during the deposition process so that a continuous, uniform coating of the metallic or metallic oxide materials results. It is important that the coating process may be controlled so that a uniform coating is deposited on the substrate and under no circumstances will any eventual blistering, cracking, peeling or spalling of the coating occur during heat treatment or during subsequent use.
In addition, the coating comprising the carrier and the selected particles suspended therein should be capable of being applied locally to a previously coated substrate to xe2x80x9crepairxe2x80x9d places during the application process where the previously applied coating has been applied too sparingly, or the coating on the surface has been impact damaged before heat treating of the coated substrate has begun.
Because the carrier for the selected particles which will ultimately become the coating is usually of a somewhat volatile nature, it must be environmentally acceptable in order to be used with this process in safety.
U.S. Pat. No. 3,102,044 Aug. 27, 1963 Joseph
This is one of the earlier coating patents, which describes a process of application of a coating mixture comprising particles of a metal or metal oxides suspended in a carrier to turbine parts.
The selected particles are suspended in a suitable dispersant which may be alcohols, esters and ketones. The carrier is evaporated by some means and the coated article is subjected to a heat treatment operation to diffuse and permanently fix the metallic dust or powder to the substrate.
U.S. Pat. No. 3,248,251 Apr. 26, 1966 Allen
This patent describes a process for coating substrates such as turbine blades with solid particulate materials such as aluminum powder carried in a phosphates/chromates/metal ion solution. The powder-carrier mixture may be applied by spraying, dipping, rolling or brushing to/on the surface of the substrate to be coated.
The coating may be then heat cured and processed to form a protective coating for the substrate.
U.S. Pat. No. 4,310,574
A lacquer slurry comprising cellulose nitrate carrying silicon powder is deposited on a substrate. The slurry was dried on the substrate and the coated substrate was placed on an aluminum powder pack and heated to 1100xc2x0 C. to produce an aluminized coating.
Articles such as turbine vanes and blades are cleaned in preparation for coating as in most of the prior art procedures. A slurry is prepared which comprises a carrier of silicone alkyd paint in which an aluminum (or an aluminum alloy) powder (in this instance Amdry 355) is mixed in the paint in a predetermined ratio ranging from 10 to 80% by weight. Additions of other elemental powders ranging from 0-15% of the mixture may also be added to alter the composition of the finished diffusion coating.
After the slurry is mixed and degassed, the viscosity is checked and if the viscosity is suitable, parts to be coated are dipped in the slurry. Parts are allowed to xe2x80x9cdrip offxe2x80x9d from selected edges or points of the dipped parts and the coating is allowed to cure for at least 60 minutes.
The coated parts may be placed in a suitable oven at a temperature less than 100xc2x0 C. to drive off the solvent materials of the carrier.
The process is completed by subjecting the partially treated coated parts to treatment at high temperature in a vacuum or inert gas atmosphere. At temperature, a reaction with the substrate to form a metal aluminum compound on the surface of the substrate occurs.